For example, high refractive index materials have been actively researched and developed in recent years to achieve thinning of the plastic lenses employed in eyeglasses and the like.
Compared to conventional glass lenses, plastic lenses are lighter and easier to process. They also afford the advantage of relatively good strength relative to impact. On the other hand, they have poorer scratch resistance and weatherability than glass lenses due to lower hardness.
Thus, particularly when applying a plastic lens as an eyeglass lens, a cured film called a hard coat layer is generally formed on the plastic lens.
Further, when applying a plastic lens as an eyeglass lens, an antireflective layer is generally formed over the hard coat layer. However, when the difference in refractive index between the antireflective layer and the hard coat layer is excessive, an interference fringe is generated. As a result, there is a need to use a material with a high refractive index to constitute the hard coat layer.
A hard coat layer formed using a hard coat material containing an organic silicon compound (silane coupling agent) and a metal oxide is a known example of a hard coat layer achieving such a high refractive index.
More specifically, the sol-gel method has been proposed (for example, see Patent Reference 1) whereby a hard coat material containing an organic silicon compound, or a hydrolysis product thereof, and a metal oxide (composite oxide sol) is prepared, the hard coat material is supplied on the plastic lens, and heating is conducted to prepare a gel, yielding a hard coat layer.
However, when a hard coat layer is formed on an eyeglass lens using such a sol-gel method, because eyeglass lenses are usually comprised of curved surfaces, the hard coat layer will be thicker at the edge of the eyeglass lens. As a result, there is a problem in that cracks are generated in the hard coat layer on the edges.